The principle of electrostatic filtering has been known for many years. Under the influence of an air stream, certain materials which are normally dielectric will obtain an electrostatic charge. The electrostatic charge attracts dust particles from the air stream, thereby filtering air. Tests have shown that self-charging electrostatic air filters provide superior filtration as compared with conventional impingement type filters which collects dust through contact with the dust particles.
U.S. Pat. No. 4,115,082 to Newell discloses an air cleaner assembly having outer sheets of polypropylene overlying two series of rigid thermoplastic rods. A central element of open-cell foam rubber is sandwiched between the two series of rods. It is claimed that the rods and central element develop a positive electrostatic charge while the outer polypropylene material develops a negative charge, increasing the efficiency of the filtration.
Because of the thermoplastic rods, the construction of the filter disclosed in U.S. Pat. No. 4,115,082 has proven to be quite expensive. Furthermore, while it is doubtful that the plastic rods can develop enough charge to significantly effect the filtration; they may increase the resistance to airflow. Independent tests have shown that the filtration efficiency of the unit of this type is only about 85% using ASHRAE 52-76 test procedures.
Therefore, a need has arisen in the industry to provide a low-cost electrostatic air filter having enhanced filtration capabilities.